LeetCode-Implement strStr()-KMP

LiBlog發表於2015-01-04
LeetCodeKMP

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Update (2014-11-02):
The signature of the function had been updated to return the index instead of the pointer. If you still see your function signature returns a char * or String, please click the reload button to reset your code definition.

Solution:

 1 public class Solution {
 2     public int strStr(String haystack, String needle) {
 3         if (needle.isEmpty()) return 0;
 4         if (haystack.isEmpty()) return -1;
 5 
 6         int[] pm = getPartialMatchTable(needle);
 7         int p1 = 0, p2 = 0;
 8         while (p1<haystack.length() && p2<needle.length()){
 9             if (haystack.charAt(p1)==needle.charAt(p2)){
10                 p1++;
11                 p2++;
12             } else {
13                 if (p2==0) p1++;
14                 else p2 = pm[p2-1]+1;
15             }
16         }
17         if (p2<needle.length()) return -1;
18         else return p1-p2;
19         
20     }
21 
22     public int[] getPartialMatchTable(String needle){
23         int[] pm = new int[needle.length()];
24         pm[0] = -1;
25         for (int i=1;i<needle.length();i++){
26             int j = pm[i-1];
27             while (j>=0 && needle.charAt(i)!=needle.charAt(j+1))  j = pm[j];
28             if (needle.charAt(i)==needle.charAt(j+1)) pm[i] = j+1;
29             else pm[i] = -1;
30         }
31         return pm;
32     }
33 }

 

相關文章